1. Field of the Invention
The present invention relates to a circuit for and a method of monitoring charge/discharge of a battery.
2. Description of the Related Art
In the battery charge/discharge monitoring circuit disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 1996-136628, two circuit systems are provided for a battery, one for monitoring its charge and the other for monitoring its discharge as shown in FIG. 7 of the publication.
The discharge monitoring circuit system activated during battery discharge has a discharge current detection resistor Rs1 provided at one of the battery terminals, detecting the potential difference between the two terminals of the resistor Rs1 with a discharge capacity detection unit. The discharge capacity detection unit outputs pulses proportional to a discharge current flowing through the discharge current detection resistor Rs1 to a discharge monitoring counter. The discharge monitoring counter counts the number of times the pulses are input from the discharge capacity detection unit, outputting the result to a judgment unit. The judgment unit judges the battery's remaining capacity based on the number of times the pulses have been input.
On the other hand, the charge monitoring circuit system activated during battery charge has a charge current detection resistor Rs2 provided at the other battery terminal, detecting the potential difference between the two terminals of the resistor Rs2 with a charge capacity detection unit. The charge capacity detection unit outputs pulses proportional to a charge current flowing through the charge current detection resistor Rs2 to a charge monitoring counter. The charge monitoring counter counts the number of times the pulses are input from the charge capacity detection unit, outputting the result to a judgment unit. The judgment unit judges the battery's charge capacity based on the number of times the pulses have been input.
The conventional circuit disclosed in the Japanese Patent Application Laid-Open Publication No. 1996-136628 is provided with two circuit systems, one for monitoring discharge and the other for monitoring charge, thus resulting in a larger circuit in scale, which is not preferred in terms, for example, of downsizing and manufacturing cost.
For measuring the battery capacity change with extreme precision, it is necessary to detect an extremely small voltage change on the order of several millivolts. However, since charge monitoring and discharge monitoring are basically implemented respectively by different circuit systems, the ground level varies differently in one system from the other. This difference in ground level leads to an innegligible error in detecting an extremely small voltage change for precise judgment of the battery capacity when charge monitoring and discharge monitoring are conducted alternately. Besides, if error signal component such as common noise accumulates in both circuit systems, it becomes impossible to precisely determine the battery capacity.